A common transmission channel is used for transmission of fixed-length packets between terminals or in combination with a master station which may itself be a terminal. This transmission channel makes available a transmission capacity between terminals and the master station (uplink) and between the master station and the terminals (downlink). A method of implementing such a packet transmission has been proposed in German Patent Application No. 197 26 120.
Possible media include a wireless channel, a passive optical network, or cable distribution networks using coaxial cables and/or glass fibers. FIG. 1 illustrates the principle. Instead of a terminal, a concentrator having a plurality of terminals may also be used (an extension unit in the possession of a subscriber or a network unit in the possession of the network operator). When a terminal is mentioned below, no restriction on generality is intended. Subscribers and their terminals are labeled as 1, the common transmission medium as 2, the master station as 3 and the transmission network as 4. In known circuit-switched systems, e.g., an analog telephone or ISDN, each terminal has its own connection to a network node, and these connections may be bundled on one line or a few lines (see FIG. 2). The same thing also applies to terminals in ATM systems (ATM=asynchronous transfer mode). In the case of an ATM, such a network node 5 is an ATM multiplexer.
The main medium to be considered here is a wireless channel. Not being tied to one location, these terminals are usually notebooks or other portable devices that can be operated with batteries. Therefore, power consumption is an important sales argument for such devices.
The following embodiments have been developed for use in ATM networks, but they can very easily also be used in other networks, such as IP (Internet protocol) networks or Ethernet networks.
It is important for the network nodes to guarantee support of service goods. Since packets of different connections must be transmitted reliably at different rates, operating strategies must be introduced. To implement such an operating strategy, use of the common medium must be coordinated, but this can be accomplished only by using a master station. Such media access controls (MAC) are known (See D. Petras, A. Krämling, “MAC protocol with polling and fast collision resolution for an ATM air interface” IEEE ATM Workshop, San Francisco, Calif., Aug. 25, 1996, and D. Petras, A. Krämling, A. Hettich, “MAC protocol for wireless ATM: contention free versus contention based transmission of reservation requests” PIMRC '96, Teipei, Taiwan, October 1996, for example).
Power saving methods are used in GSM. These methods can be used very easily because GSM supplies circuit-switched communication. In other words, the only modes are “connected” and “not connected.” In the “connected” mode, data is exchanged continuously between the network and subscriber. In the “not connected” mode, the terminal must be notified at regular intervals about which base station is currently the best to receive and whether a call is arriving for a terminal.
Incoming calls are sent out by all the base stations in the roaming area—this is known as paging—and the terminal logs on with the best base station for receiving the call at the moment. The terminal does not log on until it leaves the roaming area. In this case, it must log on in the new roaming area.
There is also a power saving mode in HIPERLAN type 1 systems. Packets of variable length are also exchanged in this method. However, the MAC protocol is organized decentrally and each subscriber attempts to occupy a channel in competition with other subscribers, as in the Ethernet MAC protocol. The MAC is connectionless, so that a packet can always be expected. The address of the receiver and the length of the packet are sent at the beginning of each packet, so that each subscriber must monitor each of these packet headers at least. If the subscriber finds that the packet is not intended for it, it can go to sleep for the duration of the packet that can be derived from the header. This is relatively inefficient because longer sleep phases are not possible. In addition, the channel must always be monitored (at least the power level), even if there is currently no traffic.
European Published Patent Application No. 0 473 465 describes an arrangement for power saving operation of a mobile communication terminal. This arrangement contains a microprocessor which is used to decode messages of a paging channel and to determine whether the message is relevant for that terminal. If the message is not intended for this terminal, the terminal goes into a power saving mode between reception of the messages on the paging channel, where first a timer is started and generates a signal after a predetermined period of time has elapsed, whereupon the microprocessor goes into an active mode and receives the message on the paging channel. The base station sends a synchronization sequence for detecting the paging message and for resynchronization.
The arrangement according to European Published Patent Application No. 0 473 465 includes a decision circuit by means of which the synchronization of the terminal can be controlled with respect to the synchronization information/frame symbols received by a master station. The decision circuit occupies the controller of the terminal when the terminal goes from an active first operating mode into another operating mode, i.e., a second or third operating mode, e.g., a standby or sleep phase. Also provided is a timer, which is controlled by the decision circuit and which opens a timing window for receiving the synchronization information/frame symbols, when such information/frame symbols are to be expected, it being possible to use received synchronization information to correct the time basis of the terminal.